1. Field of the Invention
The present invention relates to an automatic gain controller, and more particularly, to an automatic gain controller outputting a gain control signal to automatically control a signal gain inputted into a signal receiver such as a digital television.
2. Description of the Related Art
A signal receiver such as a digital television has an automatic gain controller to control the gain of the inputted signal. In case the power of the inputted signal is either excessive or low, it is difficult to properly control the signal and therefore the automatic gain controller automatically adjusts the gain of the inputted signal to an appropriate level.
The automatic gain control is necessary particularly in digital communication. In the digital communication, an analog-to-digital converter samples an analog signal passed through a radio frequency channel, and the digitalized signal undergoes a digital signal processing. When an analog signal is digitalized, an input signal needs to have a sufficient amount of power in order to minimize quantizing noise. However, if the power of the signal input is excessive, the AD converter becomes saturated and a circuit processing a digital signal causes mis-operation. Therefore, the level of the input signal should be adjusted to a certain level.
The automatic gain controller is necessary not only for a digital television but for all communication devices including a mobile communication device, etc. FIG. 1 illustrates a conventional automatic gain controller, and more particularly, an automatic gain controller used for digital communication.
As illustrated in FIG. 1, a conventional automatic gain controller comprises a power estimator 11, a subtractor 13, a gain multiplier 15, an accumulator 20, and a modulator 30.
The power estimator 11 receives a digital signal outputted from an AD converter and estimates the power of the input signal. The subtractor 13 calculates the value of a difference between the power estimated by the power estimator 11 and a predetermined reference value, i.e., an error of the signal power. The calculated difference value is inputted into the gain multiplier 15 which multiplies the difference value by a constant gain K. The constant gain K multiplied by the value of the difference between the reference value and the actual power of an input signal determines the bandwidth of the automatic gain controller.
The accumulator 20 comprises a delay unit 23 to delay the signal input and an adder 21 to add the output of the delay unit 23 and the output of the gain multiplier 15 and input the outcome into the delay unit 23. The output value of the gain multiplier 15 is accumulated by the accumulator 20.
The modulator 30 modulates the output value of the accumulator 20. A Σ-Δ DA converter generating a pulse width modulation signal corresponding to the output of the accumulator 20 is used as a modulator 30. The output of the modulator 30 is provided to other parts in a signal receiver, such as an IF amplifier or an RF amplifier requiring a gain control signal.
However, the above conventional automatic gain controller has a problem that the signal level of the input signal might not be properly adjusted when the status of the channel receiving the input signal changes too much since the gain value K controlling bandwidth is fixed. For example, as illustrated in FIG. 2, if the gain value K is set too low (K<1), it is difficult to follow the change of the input signal as the gain of the automatic gain controller becomes low, and if the gain value K is set too high (K>1), a jitter occurring at the state of the input signal being stably received may become high because the gain of the automatic gain controller becomes high.